MKC

I have a MY99 UK Turbo. A few days ago I started getting a drone/whine from 40mph upwards. Its got louder now. At 60mph and above its a constant whine unaffected by the gear im in, clutch going in and out, road conditions, steering wheel position etc. Its not linked to engine or engine speed either. Purely linked to the final road speed.

There are no clicking sounds and drive seems to feel as it always does.

Ive checked gear oil and its at its normal level. Gearchange is sweet as usual.

The sound is coming from the front centre of the car up into cockpit.
Its definitely not as loud outside the car or sitting in the back.

Can someone who knows what they are talking about please let me know what (centre dif, viscous coupling, blah, blah...) this may be and if there are any tests I can do to isolate what may be causing it.

Any example costs for Subaru fixing similar problems on anyone elses cars would also be useful. Should I keep driving it ?

WHAT'S THE WORSE, MOST EXPENSIVE THING IT COULD BE ?

Thanks.